Radio transmission systems which comprise a transmission chain having a power amplifier exhibit distortion in regard to the linearity of the signal in the operating span of the power amplifier. Moreover, in order to increase the efficiency of the power amplifier, provision is conventionally made to use it in a zone of its operating span which is close to a saturation zone. Now, this distortion is all the more significant in such an operating zone.
Such distortion degrades the frequency spectrum of the signal transmitted and therefore may disturb transmission in adjacent channels (‘ACPR’, standing for ‘Adjacent Channel Power Ratio’).
To avoid this distortion, provision may be made to distance the operating point from the saturation zone of the power amplifier. However, in this case, the efficiency of the latter is thereby diminished. Therefore, the consumption of the transmission chain increases and its energy self-sufficiency is thereby restricted.
Provision may also be made to use a linearization procedure consisting in applying a predistortion of the modulated signal before providing it to the power amplifier, the predistortion then making it possible to obtain at the output of the power amplifier the signal that one wishes to transmit corrected for the effects of distortion, in particular distortions of the output amplitude as a function of the amplitude of the input signal and distortions of the phase of the output signal as a function of the amplitude of the input signal.
Such linearization procedures are based on determining a plurality of parameters that are intended to make it possible to determine the predistortion which should be applied to the signal to be provided to the power amplifier. Such a determination is as a general rule complex. Additionally, conventionally, these linearization procedures are based on an initialization or calibration phase in the course of which initial predistortion coefficients are determined. Next, through feedback action, the predistortion coefficients are determined in a tailored manner so as to adjust their respective values as a function of modifications which may occur over time.
Thus, these procedures have the drawback of managing predistortion coefficients which are complex to determine, their determination requiring a prior calibration phase.